A radio frequency identification (RFID) tag usually comprises ESD (electro static discharge) structures on its antenna terminals in order to avoid over voltage conditions which could damage an integrated circuit (IC) of the RFID tag. The ESD structure is a voltage clamp which limits the voltage between two terminals to a predefined maximum voltage. For example, such a voltage clamp comprises diode structures which short-circuit two terminals if the voltage between the two terminals exceeds a predefined maximum voltage. Thus, electric current flows over the diode structures from one terminal to the other terminal avoiding a damage of the IC due to a high current or voltage.
FIG. 4 shows a typical radio frequency interface circuit for a RFID tag in detail. An ESD structure 22 serves as a voltage clamp and couples two input terminals RF+ and RF− for connecting the shown circuit with an antenna structure (not shown) of the RFID tag. If the voltage between the two terminals RF+ and RF− exceeds a predefined clamp voltage, the ESD structure 22 conducts such that electric current flows from terminal RF+ to RF− or vice versa. A rectifier 15 connected on its input side to the two terminals RF+ and RF− is protected by the ESD structure 22 from high electrostatic discharging voltages. A limiting transistor 23 and a modulation transistor 24 are connected in parallel to the outputs of the rectifier 15 and serve as modulation and voltage regulation circuit, respectively. The limiting transistor 23 is controlled by voltage control means 16 which are supplied by the rectifier 15. The modulation transistor 24 is controlled by modulation control means 17 which are also supplied by the rectifier 15. This known circuitry has the drawback that the ESD structures usually applied for such circuitry has a large parasitic capacitance. A further drawback is that the rectifier 15 must be designed to deliver a high current for the limiting transistor 23.
U.S. Pat. No. 5,815,355 discloses a modulation compensated clamp circuit for a RFID tag which comprises a shunt transistor connected between two input terminals for connecting with an antenna of the RFID tag. A further load transistor is connected in parallel to the shunt transistor between the two terminals and used to vary the Q factor of a tank circuit depending on data read from a memory. Both transistors are placed before a rectifier for supplying the electronic circuitry of the RFID tag with electric power. However, the two transistors connected in parallel between both input terminals still cause a high input capacitance which is not desired since it degrades the radio frequency performance. Particularly in the UHF (ultra high frequency) band, which is often used for RFID transponder, a low input capacitance plays a significant role in order to detect also low signals from a RFID reader.